Refrigerant evaporators are used in various air conditioning and heat pump systems. When air is being cooled, partially expanded two-phase refrigerant enters the evaporator where it expands to absorb heat from the air. Due to the large mass differences between the liquid and gas phases, momentum and gravity effects can result in an undesirable separation of the phases and cause poor refrigerant distribution which leads to uneven temperature distribution across the evaporator. In order to keep the phases of refrigerant well mixed, the addition of a distributor (e.g. an inlet distributor or an outlet collector) tube within a header has been proposed. However, such distributors undesirably restrict the flow of refrigerant.